新概念英語第二冊的第77課中的一句話

題目：

新概念英語第二冊的第77課中的一句話
The operation,which lasted for over four hours,proved to be very difficult because of the hard resin which covered the skin.
proved to be very difficult,to在這裡是作什麼用,另外be加形容詞是不是表示一種狀態?
另外那課後練習：It was difficult because of the hard resin ____the skin.這裡爲什麼是covering,而不是covered?

解答：

The operation是主語,謂語部分是proved to be very difficult ,which lasted for over four hours爲非限定性定語從句,to在這裡是不定式的符號,短語prove to do something.BE +形容詞表示性質、狀態.之所以用covering不用covered是因爲這裡應該使用現在分詞做後置定語,表主動含義,而用過去分詞做定語表被動含義.It was difficult because of the hard resin covering the skin.=It was difficult because of the hard resin that/which covered the skin.（持續4個多小時的手術很難是因爲覆蓋在皮膚上的樹脂）注意是說 the hard resin covered the skin.It was difficult because of the hard resin covered the skin.=It was difficult because of the hard resin that was covered the skin.注意the hard resin was covered the skin.這個句子是不成立的,應該說the hard resin was covered by/with the skin.但是句意又是不正確的.整句的意思成了手術很難是因爲樹脂被皮膚覆蓋.